Method and device for continuously treating waste water of industrial origin by water vapour stripping

ABSTRACT

The invention concerns a method characterised in that it consists in: determining online at least an ultraviolet spectrum portion of at least one of the compounds present in one or the other of the waste water feeding circuits ( 7, 13 ) or of the column ( 8 ) waste water output ( 16 ); qualitatively and quantitatively determining, by mathematical processing of the measured intensity levels, at least a contaminant present in the samples; on the basis of the results obtained and by comparing with pre-defined setpoint values, in restoring said data in the form of electric signals, which control the waste water supply and the stripping column water vapour flow rates.

[0001] This disclosure is based upon, and claims priority from, Frenchpatent application No. 15568 filed Dec. 1, 2000, the contents of whichare incorporated by reference herein.

[0002] The present invention concerns a method and device for continuousprocessing by water vapor stripping of industrial wastewater that maycontain contaminants, that is, by extraction of the most volatile partscontained in said water.

[0003] It is known that in all large industrialized countries, the lawsimpose standards that are stricter and stricter and more limitingconcerning the quality of wastewaters produced by various industrialfacilities, which are disposed of into the, natural environment. Indeed,said waters may have been contaminated by various pollutants that arehazardous to humans and the environment when they are used, and it istherefore necessary and essential to give them appropriate processingbefore returning them to the natural environment.

[0004] Thus, in oil refineries, which constitute a type of industrialfacility that will be referred to more specifically in the followingdescription, without however being limited thereto, the waters called“process waters,” originating from the different units of the refinery,represent the majority of waters that must undergo processing beforethey are reintegrated into their initial environment. Indeed, beforerunning these waters through various physical and biological finishactions, they are first given a primary application called strippingwhich makes it possible to extract the most volatile parts they containby physical action. It is for this purpose that an oil refinery isusually equipped with at least one stripping device, called “stripper”in the industry, in which the wastewaters produced by the differentprocesses are given a counter-current treatment by a flow of water vaporin a column housing several horizontal plates.

[0005] The process waters treated in this way, which contain onlylimited quantities of a number of pollutants such as ammonium hydroxide,sulfides, or various phenols, are evacuated to the lower part of thestripper, then mixed with runoff water, and are finally given varioustreatments called finishing, before being discarded into the naturalenvironment.

[0006] Among these treatments are:

[0007] a pretreatment by decantation with surface skimming to eliminatedroplets of insoluble hydrocarbons and the largest particles ofsuspended particulate matter (SPM);

[0008] a physical-chemical treatment of flocculation and filtration toeliminate most of the residual insoluble hydrocarbons, the smallsuspended particulate matter and metals that are dissolved or insuspension;

[0009] a filtration treatment, such as a trickling filter, in particularto reduce the organic load (total organic carbon, or TOC), theconcentration in phenols and the concentration in dissolvedhydrocarbons;

[0010] a final clarification treatment.

[0011] In a refinery, the stripper placed upstream of these additionaltreatments therefore has an essential function, because it receivesnearly all of the waters produced by various sources (desalting of crudeoils, distillation units, gasoils desulfurization unit, cat crackers,etc.), heavily contaminated by, among other things, gases as well astoxic and malodorous compounds.

[0012] A stripper is generally composed of a column inside which platesare arranged at several levels and in which the waters to be processedare introduced at the column head and the water vapor at the bottom.Currently on the market there are several variations of strippers,particularly those:

[0013] without column head condenser (the simplest);

[0014] with head condenser and control of the reflux rate;

[0015] with head condenser and bottom reboiler, thus replacing theinjection of the water vapor flows;

[0016] with two superimposed columns for better efficiency of strippingof heavily contaminated waters.

[0017] In order to limit and protect the additional treatments of watersexiting the stripper, and consequently to prevent or limit, for example,the deterioration of the bio-filter used in one of these treatments, aswell as to obtain maximum stripping effectiveness, it is thereforeessential and even indispensable that the stripper work under the bestoperating conditions.

[0018] In order to optimize this operation, the operator can deal withthe following parameters:

[0019] the feed flow rate of stripping vapor,

[0020] the feed flow rate of wastewater to be treated, and, for certainstripper technologies:

[0021] the reflux rate of the water at the column head,

[0022] the reboil rate at the column bottom.

[0023] Each type of stripper is selected in accordance with thespecificities of the industrial sites and the treatments to beperformed, according to the standards in force for wastewaters, whichmay have a greater or lesser load of contaminants.

[0024] Because the adjustment of these parameters is directly related tothe nature and concentrations of the contaminants present in the processwaters, the refiner must be able to have such information at all timesin order to optimize the operation of the stripper. At present, however,there is no satisfactory method for the instantaneous orquasi-instantaneous analysis of the composition of contaminants and thedegree of pollution of the wastewaters arriving at the stripper. Theonly methods of analysis available today are laboratory methods, whichresult in major delays in response and numerous ways in which humaninvolvement is required. The water samples must be taken, sent to thelaboratory, then analyzed by different methods of analysis, some ofwhich require special preparation of the sample. Among these methodsare:

[0025] potentiometric quantitative analysis for measuring sulfides,

[0026] distillation, then calorimetric titration for ammonium hydroxide,

[0027] high performance liquid phase chromatography for measuringphenols,

[0028] UV oxidation, linear IR detection for Total Organic Carbon.

[0029] On the site of operations, at best it takes many minutes, evenseveral hundred minutes after the samples are taken, until the refinerhas results from the analysis, and then only with limited frequency,such as fewer than five times per 24-hour period of operation of thestripper.

[0030] The present invention therefore proposes to remedy thedisadvantages of these different methods of laboratory analysis of theformer technology by proposing a simultaneous analysis of the differentpollutants usually present in industrial wastewater, using a singleanalysis method, ultraviolet spectrometry, directly and automaticallyapplied to the feed water and/or output water of the stripper.

[0031] A purpose of the invention is also to have the results of theanalyses nearly in real time, for continuous control of the stripper,using means already described in the technology, by controlling andregulating certain principal parameters of the stripper's operation,such as the feed flow rate of wastewater to be treated, and the flowrate of stripping water vapor.

[0032] To that end, a purpose of the present invention is a method forcontinuous processing of industrial wastewater that may contain variouscontaminants, according to which the wastewater is introduced by atleast one feed line into a stripper column in which it flows by gravity,a flow of water vapor is injected into this column at a level such thatthe wastewater and the water vapor circulate in counter-current in thecolumn, the gases extracted by this vapor stripping of the wastewaterare recovered at the column head, and at the base of the stripper columnthe treated water is evacuated, this method being characterized in that:

[0033] at least a portion of the ultraviolet spectrum of the compoundspresent in one or the other of the wastewater feed circuit or the outletcircuit of treated water from the column is determined on line;

[0034] by mathematical processing of the measured intensities, at leastone contaminant product present in the samples taken is determinedqualitatively and quantitatively;

[0035] depending on the results thus obtained, and by comparison topreviously defined set points, this information is retrieved in the formof electrical signals that control the flow rates of the wastewater andwater vapor feeds in the stripper column.

[0036] The taking of water samples, the measurements made in the domainof the ultraviolet light spectrum, the transmission of the electricalsignals and the control of the operational units controlling the flowrates of wastewater and water vapor feeds are preferably performed by atleast one programmed, automatic control device.

[0037] The analysis technique, which consists of:

[0038] 1. using a commercial spectrometer to measure the UV spectrumfrom a sample, or a portion of this spectrum, or a specific wavelengththereof, whether the spectrum is emitted directly or by fluorescence, orabsorbed by this sample, when the sample is excited by a source emittingultraviolet light,

[0039] 2. then applying to the intensities thus recorded, a mathematicalprocessing, such as the deconvolution of spectra or PLS (partial leastsquare), is well known to a person skilled in the art.

[0040] For example, reference can be made to the following twopublications:

[0041] O. Thomas, F. Theraulaz, C. Agnel, and S. Suryani (“Advanced UVexamination of wastewater; Environmental Technology,” 1996, vol. 17, pp251-261);

[0042] O. Thomas, F. Theraulaz, M. Domeizel, and C. Massiani (“UVspectral deconvolution: a valuable tool for wastewater qualitydetermination,” Environmental Technology 1993, vol. 14, pp 1187-1192).

[0043] This analysis technique that is known for its use in thelaboratory and which consists of using UV spectrometry to measure thesulfides, ammonium hydroxide, or certain phenols in water, has requiredvarious adaptations in order to apply it to a continuous processdirectly on the circuits of the stripper.

[0044] Thus, for example, the presence of undissolved hydrocarbons inthe process waters very quickly causes a fouling of the sampling lineand of the measuring cell, disrupting the UV measurement. Consequently,to remedy this disadvantage, a coalescer should be installed at the headof the sample feed line, in order to keep only the aqueous phase of thesample for UV analysis.

[0045] Moreover, because the pH of process waters usually varies between7 and 10, it is known that at pH 4 the sulfides in the form of H₂S arenot observable by UV spectrometry. In order to ensure an exhaustiveanalysis of these sulfides, the water sample must therefore be dilutedwith an aqueous buffer solution with a high pH, such as 10, so thatregardless of the value of the pH of the sample to be analyzed, itsdilution with the buffer solution will bring its pH back above 8, andpreferably between 8 and 10, in which area all sulfides are observableand therefore measurable by UV spectrometry.

[0046] As a result of the continuous control mode of the stripper columnused in this method, it is possible to more closely monitor theconcentration(s) of the different contaminants contained in the water,for example at the outlet of the column, by automatically regulating theflow rates of wastewater feed or stripper water vapor. In addition, inthe event of an abrupt increase in the concentration of one or morecontaminants, a deviation of all or part of the flows of water to betreated upstream of the stripper can be implemented nearly in real time,automatically, to a temporary storage tank, for the possible applicationof a special treatment or to dilute said water.

[0047] The modification of the stripper's flow rates of feed water andvapor, as a function of the results of this analysis, is done in aconventional way known to a person skilled in the art.

[0048] In this regard, it will be noted that it is generally notpossible to modify other operational parameters, such as thetemperatures and pressures of the water flows entering or leaving thestripper. However, depending on the values measured by the ultravioletspectrometer placed in line upstream or downstream from the stripper,the operator can automatically or non-automatically adjust the pH of thewater for greater stripping effectiveness, while remaining withincertain acceptable limits for the quality of the water disposed of inthe natural environment.

[0049] The method, according to the invention, is applicable to anystripper associated with a refining unit (or any other industrialenterprise), in which waters are collected upstream from different unitsand/or secondary strippers that are associated therewith.

[0050] A purpose of the invention is also a device for continuousprocessing of wastewater of industrial origin, this device comprising:

[0051] a stripper column for the water;

[0052] at least a feed line of this stripper column for the water to betreated, this feed being at a level such that the water flows by gravitytoward the bottom of the column;

[0053] at least a feed line of the stripper column for water vaporintended to strip the gases contained in the water to be treated, thewater vapor feed being at a level such that the wastewater and the watervapor circulate at counter-currents in the column;

[0054] at the upper part of the column, at least a line for evacuatingthe water vapor and gases stripped from the wastewater by this vapor,with possibly a means for separating the gases and the vapor;

[0055] at the lower part of the column, at least a line for evacuatingthe treated water;

[0056] on the stripper column's wastewater and water vapor feed lines,controls for adjusting the flow rate, this device being characterized inthat it comprises:

[0057] on at least one wastewater feed line or on at least a treatedwater outlet line, a controlled means of taking a sample from the watercirculating in this line;

[0058] connected to these sample taking means, a means of analysis byultraviolet spectrometry and an associated calculation means, fordetermining the presence and quantity of any contaminants present in thesamples taken;

[0059] connected to this calculation means, a programmed device foractuating the operational components to control the flow rate of thestripper column's wastewater feed lines and water vapor lines, as afunction of the results of analysis of the samples.

[0060] As indicated above, this device can also advantageously includemeans for measuring other operational parameters, such as thetemperature and pressure of the water flows entering and leaving thestripper column, and means of transmitting these measurements to thedevice for actuating the stripper column's operational controlcomponents.

[0061] Other characteristics and advantages of the invention, in itsapplication to the treatment of wastewater from part of an oil refineryhaving an atmospheric distillation column and various associatedfacilities, will appear from the following detailed description. In thisdescription, reference will be made to the appended drawings, in which:

[0062]FIG. 1 is a partial diagrammatic view of the refinery's wastewatercircuits, illustrating the position of the main water vapor column forstripping gases contained in the wastewater;

[0063]FIG. 2 is a partial diagrammatic view illustrating the controlsystem according to the invention for the stripper column.

[0064] Reference will first be made to FIG. 1, in which, for purposes ofsimplification, only some of the primary strippers, preferablyassociated to each of the units, are shown in diagrammatic form.

[0065] The feed of crude oil and desalting water from the desalter 3 isprovided respectively by lines 1 and 2. The salt water from the desalter3 passes through the line 4 into a decanter where the liquidhydrocarbons present are evacuated by the line 6, while the salt waterrecovered by line 7 is injected into the lower part 8 a of a strippercolumn 8, which, in this example, has two stages 8 a and 8 b.

[0066] The crude oil issuing from the desalter 3 is introduced by theline 9 into an atmospheric distillation column 10. The various outletlines of the separate cross sections of said atmospheric distillationcolumn 10 are not represented.

[0067] The vapors recovered at the column head are directed by the line11 to a condensation recipient 12, where the condensed water is routedby the line 13 to the upper part 8 b of the stripper column 8.

[0068] The water feed line 2 from the desalter is connected to the line13 and various makeup feeds are introduced there, for example from astorage tank 23 of water from various origins, from a condensation tank24 of effluents from a gas oil drying column head, and from variousprimary strippers associated with units of the facility, such as astripper 25 from a gas oil desulfurization unit and a stripper 26 from acatalytic cracking unit.

[0069] In the column 8, the water introduced at 7 and at 13 trickle bygravity down toward the bottom, in counter-current to a water vapor flowintroduced at a lower level by the line 14, and the gases dissolved inthis water are stripped by the water vapor and evacuated at 15 with theremaining vapor at the upper part of the column 8. The water vapor canbe generated by a continuous reboiling device located at the bottom ofcolumn 8.

[0070] The treated water is evacuated by line 16 at the lower part ofthe column 8 and it passes successively into a decanter 17, onto sandfilters 18, into a bio-filter 19 and again into a clarification basin20, before being evacuated by line 21 into the natural environment.

[0071] The bio-filter 19 is a runoff, systematic packing aerobicbacterial bio-filter, intended to ensure the destruction bymicro-organisms of dissolved organic material, transforming it intobiomass, carbonic acid gas and water. This bio-filter also provides forthe nearly complete elimination of the hydrogen sulfide, primarily byaeration.

[0072] The bacteria of this bio-filter are very sensitive to certainpollutants, which are toxic to them above a certain concentration, equalfor example to 8 mg/l for sulfides. It is important, therefore, toeliminate these contaminants or to reduce their concentration belowthresholds upstream from the bio-filter, and in particular at thestripper.

[0073] As indicated above, one of the purposes of the present inventionis specifically to control the operation of this stripper bycontinuously measuring the concentrations of various pollutants of thewastewater to be treated and/or pollutants of treated water, and byvarying the stripper's feed flows of this water, of water vapor andpossibly of recycled condensed water vapor, in order to operate underthe most effective water purification conditions.

[0074] The following is in reference to FIG. 2.

[0075]FIG. 2 again shows the stripper column 8 of FIG. 1, the line 7feeding water into this column water that issues from the crude oildesalter, the line 13 introducing other wastewater to be treated, line14 injecting water vapor, line 15 evacuating, at the column head, thewater vapor and the gases stripped by said vapor, and line 16 evacuatingthe treated water at the bottom of the column.

[0076] In this configuration, the line 15 feeds a condensation recipient21, where the stripped gases are evacuated by the line 28, while thewater produced by the condensation of the vapor is recycled by the line29 at an adjustable reflux rate at the column head.

[0077] The lines 7, 13, and 14 have valves, respectively 7 a, 13 a and14 a, that allow the feed flow rate of the fluids circulating in thecolumn 8 to be adjusted.

[0078] According to the invention, an ultraviolet spectrometry analyzer30 is connected to the lines 7, 13, and 16, respectively, by the lines31, 32, and 33, in order to take controlled samples of fluidscirculating in these lines and to determine the concentration of certaincontaminants present in said samples. Analyzers of this type are wellknown in the technology, and for example are marketed under the name IXO510 by the SECOMAN company.

[0079] The spectra obtained are processed by spectral deconvolution in aprogrammed control center 34, that is, they are broken down into acertain number of spectra of the contaminants on which information isbeing sought (sulfides, chlorides, total organic carbon or TOC,materials in suspension or MIS, ammonium hydroxide, etc.). The spectraobtained are automatically compared to the spectra of a reference basecomposed of known samples, in order to determine the concentration ofthese contaminants in the samples. This method of analysis bydeconvolution is described, for example, by S. Gallot and 0. Thomas in“State of the art for the examination of UV spectra of waters andwastewaters,” Intern. J. Environ. Anal. Chem., vol. 52, pp 119-158.

[0080] The control center 34 includes a control program which, dependingon the analysis results, operates the valves 7 a, 13 a 14 a and thecondenser 27 to modify appropriately the feed flows of wastewater andwater vapor in the column 8, as well as the rate of reflux of water atthe column head.

[0081] The taking of samples can be controlled by an operator, or can bedone automatically at regular intervals, such as every ten minutes, bythe control center.

[0082] The great simplicity of implementation of the method according tothe invention will be noted. The length of time required for theanalyses is, generally around five minutes and the control of thestripper can consequently be carried out immediately after theseanalyses.

[0083] Although the invention has been described essentially in itsapplication to processing wastewater of a refinery, it will be clear toa person skilled in the art that it also applies to the treatment of anyother type of wastewater produced by different industrial facilities.

1. Method for continuous processing of industrial wastewater that maycontain various contaminants, according to which the wastewater isintroduced by at least one feed line (7, 13) into a stripper column (8)in which it flows by gravity, a flow of water vapor is injected (14)into this column (8) at a level such that the wastewater and the watervapor circulate in counter-current in the column (8), the gasesextracted (at 15) by this vapor stripping of the wastewater arerecovered at the column head, and at the base of the stripper column (8)the treated water is evacuated (at 16), this method being characterizedin that: at least a portion of the ultraviolet spectrum of the compoundspresent in one or the other of the wastewater feed circuit (7, 13) orthe outlet circuit (16) of treated water from the column (8) isdetermined on line; by mathematical processing of the measuredintensities, at least one contaminant product present in the samplestaken is determined qualitatively and quantitatively; depending on theresults thus obtained, and by comparison to previously defined setpoints, this information is retrieved in the form of electrical signalsthat control the flow rates of the wastewater and water vapor feeds inthe stripper column.
 2. Method according to claim 1, characterized inthat the taking of these water samples, their analysis by ultravioletspectrometry and the transmission of the control signals for setting andadjusting the flow rates of wastewater and water vapor feeds of thestripper column (8) are controlled by at least a programmed, automaticcontrol device (30, 34).
 3. Method according to either of claims 1 and2, characterized in that the contaminants measured on line in the waterof the feed circuit (7, 13) and outlet circuit (16) of the stripperbelong to the group including sulfides, ammonium hydroxide, and certainphenols.
 4. Method according to claim 3, characterized in that themeasurement by UV spectrometry is made on the aqueous portion of thesample, after separation from its organic phase.
 5. Method according toeither of claims 3 and 4, characterized in that the sample is diluted,prior to being measured by UV spectrometry, in a buffer aqueoussolution, to bring its pH to a value higher than 8, and preferablybetween 8 and
 10. 6. Method according to any of claims 1 to 5,characterized in that the pH of the water to be treated is adjusted inaccordance with the results of the UV spectrometry measurements. 7.Method according to any of claims 1 to 6, characterized in that thewastewater to be treated is introduced into the stripper column at atleast two different levels (at 7 and at 13) thereof.
 8. Method accordingto any of claims 1 to 7, characterized in that the gases extracted bystripping from the wastewater are separated from the water vapor in acondensation recipient (27) and evacuated (at 28) for possiblesubsequent treatment.
 9. Method according to claim 8, characterized inthat the water produced by the condensation of the vapor in thecondensation recipient is recycled (at 29) at the column head (8) withan adjustable rate of reflux.
 10. Method according to any of claims 1 to9, characterized in that the vapor introduced at the base of the column(8 a) is generated by a continuous reboiling device located at thebottom of the column (8).
 11. Method according to claim 1 to 10,characterized in that the wastewater to be treated is introduced intothe lower part (8 a) of the stripper column and in the upper part (8 b)thereof.
 12. Device for continuous processing of wastewater ofindustrial origin, this device comprising: a stripper column (8) for thewater to be treated; at least a feed line (7, 13) of this strippercolumn (8) for the water to be treated, this feed being at a level suchthat the water flows by gravity toward the bottom of the column; atleast a feed line (14) of the stripper column (8) for water vaporintended to strip the gases contained in the water to be treated, thewater vapor feed being at a level such that the wastewater and the watervapor circulate at counter-currents in the column (8); at the upper partof the column (8), at least a line (16) for evacuating the water vaporand gases stripped from the wastewater by this vapor, with possibly ameans for separating the gases and the water vapor (27); at the lowerpart of the column (8), at least a line (15) for evacuating the treatedwater; on the stripper column's wastewater and water vapor feed lines(7, 13, 14), controls (7 a, 13 a, 14 a) for adjusting the flow rate;this device being characterized in that it comprises: on at least awastewater feed line (7, 13) or on at least the treated water outletline (16), a controlled means of taking a sample from the watercirculating in this line; connected to these sample taking means, ameans (30) of analysis by ultraviolet spectrometry and an associatedcalculation means, for determining the presence and quantity of anycontaminants present in the samples taken; connected to this calculationmeans (30), a programmed device for actuating the operational componentsto control the flow rate of the stripper column's wastewater feed linesand water vapor lines, as a function of the results of analysis of thesamples.
 13. Device according to claim 12, characterized in that itincludes a means of programmed control of the means of taking samples ofthe wastewater and the treated water.
 14. Device according to either ofclaims 12 and 13, characterized in that the stripper column (8) has twodifferent stages (8 a, 8 b) each receiving a portion of the water to betreated.
 15. Device according to any of claims 12 to 14, characterizedin that a condensation recipient (21) is located at the head of thecolumn (8), in order to separate the stripped gases and the water vapor.16. Device according to claim 15, characterized in that the reflux rateat the column (8) head of the condensation water from the water vapor isadjustable.
 17. Device according to claims 12 to 16, characterized inthat a reboiler is located at the bottom of the column (8), in order totransform the condensation water into water vapor.
 18. Use of the methodaccording to any of claims 1 to 11 or of the device according to any ofclaims 12 to 17 for treating wastewater from an oil refinery.